The present invention relates to the medical diagnostic imaging arts. It finds particular application in conjunction with a dual reference blacklevel clamping device and method for reducing video line noise from an image receptor associated with a diagnostic imaging system, and will be described with particular reference thereto. However, it should be appreciated that the present invention may also find application in conjunction with other types of imaging systems and applications where reducing video line noise is desirable.
The sensitivity of all x-ray image detector devices, including flat panel image sensors or receptors, is limited by noise, i.e., random fluctuations in signal that are competing with data or other information that represents or otherwise defines a captured image. One type of noise that is characteristic of some flat panel image receptors, such as amorphous Silicon-based, flat panel, image receptors, is line correlated noise. Line correlated noise can be defined as random fluctuations that affect a whole raster line of a video frame in a manner that causes all the picture elements ("pixels") of a raster line to commonly deviate from their actual captured image values.
The manifestation of line correlated noise in a video image displayed on a video monitor is stripes that fluctuate in intensity across the width of the image. This is an undesirable effect that is highly distracting to medical personnel, such as physicians, using the imaging system to perform critical work, such as live interventional procedures.
There are known image processing techniques, generally referred to as blacklevel clamping or line noise clamping, for reducing the amount of line correlated noise generated in image receptors. These techniques rely on a single, vertically elongated (i.e., perpendicular to the image raster lines), clamp or reference zone along either side edge of an image frame or array that is "blacked out" from receiving or capturing image information. The image information can be visible light in the case of a video camera, or the light output from a scintillator screen, in the case of a fluoroscopy system.
The "blacked-out" reference zone is a predetermined number of pixels wide (typically 1 to 256 pixels in length). The only output signals or "information" received from the pixels within the "blacked-out" reference zone is random noise and line correlated noise--the same line noise that affects the pixels in the exposed or "active" region of the image receptor.
Line noise cancellation involves calculating the average output value of the reference pixels for each raster line of each video frame to average out the random noise fluctuations of each reference pixel, and to yield the error introduced in that particular raster line by line noise alone. The resulting error value is then uniformly subtracted from each of the "active" pixels in the corresponding raster line prior to being displayed on a video monitor.
The known blacklevel clamping or line noise clamping techniques neglect the fact that, for some image receptor implementations, line correlated noise may not be uniform across the entire raster line. That is, the line correlated noise that is summed with the received or sensed image information may have a random slope or random profile across the image width. In these cases the conventional clamping techniques will not yield satisfactory results, as the calculated error due to line noise will only be valid in or near the reference zone. Thus, known blacklevel clamping or line noise clamping techniques yield imperfect line noise cancellation in those image acquisition systems where line correlated noise has a non-uniform profile across an image width.
Accordingly, it has been considered desirable to develop a new and improved dual reference blacklevel clamping device and method for reducing video line noise from an image receptor which meets the above-stated needs and overcomes the foregoing difficulties and others while providing better and more advantageous results.